This application relates generally to receiver circuits and, in particular to a CATV tuner with a frequency plan and architecture that allows the entire receiver, including the filters, to be integrated onto a single integrated circuit.
Radio receivers, or tuners, are widely used in applications requiring the reception of electromagnetic energy. Applications can include broadcast receivers such as radio and television, set top boxes for cable television, receivers in local area networks, test and measurement equipment, radar receivers, air traffic control receivers, and microwave communication links among others. Transmission of the electromagnetic energy may be over a transmission line or by electromagnetic radio waves.
The design of a receiver is one of the most complex design tasks in electrical engineering. In the current state of the art, there are many design criteria that must be considered to produce a working radio receiver. Tradeoffs in the design""s performance are often utilized to achieve a given objective. There are a multitude of performance characteristics that must be considered in designing the receiver. However, certain performance characteristics are common to all receivers. Distortion and noise are two such parameters. The process of capturing the signal creates distortion that must be accounted for in the design of the radio receiver. Once a radio signal is captured, the noise surrounding the received signal in the receiver must be considered. Radio signals are often extremely weak and if noise is present in the circuit, the signal, even though satisfactorily received, can be easily lost in this noise floor. The current state of the art in receiver design is often directed to overcoming these receiver limitations in a cost effective manner.
There is therefore provided in an exemplary embodiment of the present invention an oscillator having low phase noise or jitter and high isolation, that substantially increases the performance of a tuner architecture integrated onto a single silicon substrate.
The exemplary oscillator circuit includes a resonator coupled across the terminals of an active network, an active network being a circuit containing passive and gain elements. A linear buffer amplifier is coupled to the and at least one nonlinear buffer amplifier is cascaded with the linear buffer amplifier""s output for producing a differential output signal.
To implement an oscillator with low phase noise according to an embodiment of the present invention, a resonator circuit is constructed according to any typical means known in the art, including a parallel resonant circuit or a series resonant circuit, or any circuit that exhibits a self resonant frequency. The resonant circuit develops a voltage across two terminals such that at the resonant frequency there is a very high input impedance present. At frequencies above or below resonance the impedance that is much less than at the resonant frequency. The resonator circuit is connected to an active network. The active network contains an active device, typically a transistor, which provides gain and feedback to the resonator. When the Nyquist criteria mentioned earlier are satisfied for gain and phase, the combination of active network and resonator oscillate at a sinusoidal frequency determined by the resonant frequency of the resonator circuit. A linear buffer amplifier is coupled to the resonator and active network junction. The linear buffer amplifier provides a high input impedance. This high impedance prevents the active network and resonator combination from being loaded. Loading would cause the frequency of oscillation to shift. The linear buffer amplifier provides an exact reproduction of the signal input to it at a higher output power level. Because the buffer is linear, very little distortion is introduced to the signal that is amplified through it. The output of the linear buffer amplifier is cascaded with a nonlinear buffer amplifier. The nonlinear buffer amplifier transforms the sinusoidal output of the linear buffer into a square wave output. This output is a differential signal that exhibits high common mode noise rejection.